The field relates to compact, ported cylinder constructions for two-stroke cycle engines in which the bore diameter of a ported cylinder transitions in the vicinities of the ports to accommodate the passage of piston rings across the ports.
A cylinder for a two-stroke engine may be constructed by boring an engine block or by inserting a liner (also called a sleeve) into a cylindrical space formed in an engine block. The following description presumes a cylinder with a liner construction; however the underlying principles apply as well to a bored construction. For ease of discussion, the terms “cylinder” and “cylinder liner” are used interchangeably.
A cylinder liner of a two-stroke cycle, opposed-piston engine has a bore and intake and exhaust ports located near respective outer extremities of the liner. Each port has bridges separating multiple port openings that are arrayed along a respective circumference of the bore. A cylinder liner so constructed forms a “ported cylinder” when received in an engine. A ported cylinder raises at least two technical challenges to the durability and operation of an opposed-piston engine.
While the engine runs, the bridges of both ports expand inwardly as the cylinder liner thermally cycles during two-stroke operation. Inward expansion of the bridges increases frictional contact of the bridges with piston crowns, which leads to excessive compression ring wear, increased blow-by, and undesirable oil consumption as the pistons traverse the ports. One way to mitigate the effects of expansion of the port bridges is by provision of a cylinder bore construction in which the bore diameter is changed from a first size in central and end portions of the bore to a second, greater size in longitudinally-separated, annular areas of the bore where the bridges are located. Since engine specifications such as compression ratio are referenced to the first diameter, it may also be referred to as the “standard” (or “spec”) diameter. Each of the annular areas contains the ends of the bridges and the openings of one of the ports. The recess formed in an annular area by the larger diameter is referred to as a “belly”. A belly construction allows port bridges to expand inwardly as the cylinder liner thermally cycles, without protruding into bore space within the standard diameter.
In a ported cylinder of an opposed-piston engine, the bore is lubricated by oil splashed into the piston/bore interfaces at the ends of the cylinder, from where it is transported further into the bore on the external surfaces of the piston skirts and compression rings as the pistons advance toward top dead center locations during a compression stroke. It is desirable to limit the oil deposited on the pistons' skirts and compression rings to an amount sufficient to lubricate the piston/bore interface. Excess oil carried in this manner across the exhaust and intake ports will mix with scavenging air in the combustion space, reducing combustion efficiency, fouling the piston crowns, and producing undesirable exhaust components. Unburnt oil will also mix in the exhaust gasses, further contaminating the combustion products. One means of removing excess oil from the piston/bore interfaces in a ported cylinder of an opposed-piston engine is provision of oil scraper rings that act between the lower portions of the skirts and the end portions of the cylinder bore between the ports and the open ends of the cylinder. In a preferred construction, an oil scraper ring is seated in an annular groove on the lower portion of a piston skirt, and wipes excess oil from the bore as the piston approaches bottom dead center near the end of an expansion stroke.
In a ported cylinder with a belly construction, it is not desirable for the oil scraper rings to traverse the ports. At the end of a compression stroke, when the pistons are at top dead center, the ports have cooled sufficiently for the annular areas comprising the bellies to contract, thereby increasing the bore diameters in the bellies. If the oil compression rings traverse the ports, the gaps in the oil control rings can open circumferentially into the annular area due to the change in bore diameters, thereby increasing the risk of ring wear and, possibly, failure. This restriction on the locations occupied by the oil scraper rings results in a longer cylinder. Addition of the extra length for two opposed pistons to the longer cylinder length dictates smaller connecting rod angles, which for some opposed-piston architectures results in longer connecting rods and an increase in overall engine length.
It therefore would be advantageous to reduce the overall length of the ported cylinder without risking oil consumption and damage to the oil scraper rings and reducing performance and durability of the engine. The following specification describes a ported cylinder construction that leads to a reduction in cylinder length while at the same time providing friction reduction in the bore, reduced liner and piston packaging, and overall reduction in the engine dimension that corresponds with the cylinder length.